With Apple darting gingerly above the $1 trillion market cap territory today after crossing it yesterday for the first time ever following some early morning weakness today, traders are asking if there is only upside now to Apple, especially with tens of billions more to go on the company's stock buyback program.

But maybe $1 trillion, while a nice round number, is not the real resistance level: as Bloomberg's Michael Regan writes, "whether or not the round number gives investors pause is open for debate, but there's a coincidental stumbling block to consider."

Regan is talking about Apple's increasingly weight as a share of the entire stock market: to wit, the latest surge has put it above 4% of the market cap of the S&P 500. And as Regan notes, while Apple was able to stay above that weighting for much of 2012, its outperformance has petered out in subsequent years as it approached or hit that threshold:

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Why could 4% of S&P prove to be a bigger peak for AAPL to surmount than the $1 trillion market cap, which has largely been scaled thanks to hundreds of billions in debt-funded buybacks? Because as the Bloomberg commentator notes, "surmounting a 4% share of the index may be a bigger challenge considering that active fund managers may not be too comfortable being "overweight" a stock that's already such a huge weight."

The other reason: virtually everyone - hedge funds, mutual funds, pension funds, reserve managers and even central banks...

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... are already long to the gills AAPL stock. Which means that aside from the company buying back its own stock, the question is who is left?